1. Technical Field
This invention relates to a method and apparatus for producing power, and more particularly, to a method and apparatus for producing ultra-reliable power with redundancy which requires little maintenance or supervision and with improved fuel consumption.
2. Background of the Invention
The requirements for reliable power supply are more and more stringent with the advance of modern industry, computer and telecommunications systems and with the increasing costs of non-supply electricity. This particularly applies for on-site generation of electricity, be it for grid connected and distributed generation or off-grid prime power supply at remote locations. The conventional solutions for providing high efficiency on-site generation of electricity include, for short periods of interruption, battery or flywheel uninterruptible power supply (UPS) systems are used; and for longer periods, engine driven generators such as diesel generators are used for both applications relying on grid power, as well as for distributed generation and off-grid applications. The short term standby power using batteries has a distinct disadvantage when failures occur. This is especially true since there is no satisfactory diagnostic system to detect failures of batteries on standby: a single cell failure can cause failure of the whole battery pack. Expensive climate control and limited life are also drawbacks of battery systems. In addition, while flywheel systems do not have the diagnostic problem, the systems can support the load for even less time than the battery systems.
The diesel generators used for longer periods of standby operation, however, have problems of unreliable startups and require frequent maintenance and periodic overhaul. Fuel cells and stirling engines can also be used but fuel cells have too long of a startup process and these systems are still in the development stage and have no proven reliability.
Combined cycle power plants, on the other hand, i.e., a power plant having usually a gas turbine and a bottoming cycle power generating unit, have a quite high overall efficiency since heat contained in the exhaust gases of the primary power generating unit is utilized in the bottoming cycle power generating unit to produce electric power. However, the reliability of such systems can be questionable. For example, see the article “Raising the Reliability of Advanced Gas Turbines”, Vol. Power, Vol. 146, No. 2, March/April 2002, which reports that there are several reliability issues that need to be addressed when using combined cycle power plants.
For power generation systems that supply remote telecommunications with high reliable off-grid power, several options are available including: multiple diesel generators (MDG), photovoltaics, photovoltaics combined with diesel generators, thermoelectric generators (TEG), and closed cycle vapor turbogenerators (CCVT).
Multiple diesel generators (MDG) with one generator operating and one or two generators on standby has an advantage in that these systems have low fuel consumption and can operate using liquid or gaseous fuel. A multiple diesel generator system, however, depends on the reliability of the start-up of a standby generator if the operating generator fails. This necessitates a large battery to be included in the system so that it can be used in the event that the standby generator does not start. Further, the included large batteries typically require climate control in the form of heating or air conditioning, thus increasing the complexity and fuel consumption of the system for a given load.
In photovoltaic systems, batteries are used to compensate for the hours/days without solar radiation. Batteries in photovoltaic systems are usually quite large and work on deep discharge cycles. Because of the deep discharge cycles nickel-cadmium are better suited in photovoltaic systems than lead acid batteries. The cost of nickel-cadmium batteries is very high. In addition to the high cost of batteries, the life-span of these batteries is usually less than 10 years. Maintenance, vandalism and theft of the batteries of photovoltaic are additional concerns for photovoltaic systems.
Photovoltaic systems having a diesel generator back-up share the same problems as the previously-mentioned strictly photovoltaic systems, namely, cost, life-span and maintenance of batteries as well as risk of vandalism and theft. In addition, utilizing a diesel generator as a back-up power source can produce reliability issues as the system redundancy depends on an unreliable diesel engine start.
As far as thermoelectric generators (TEG) are concerned, the TEG system has the highest fuel consumption of any of the systems thus far discussed. The high fuel consumption is aggravated by the fact that a TEG system is a constant power device that requires a dummy load for dissipating any excess energy and, thus requiring additional fuel consumption due to the over-sizing of the unit, output variations due to ambient conditions or varying load requirements. If additional batteries are not used, the battery will not be properly charged and will require additional maintenance and manual charging during maintenance and thus the life span of the battery will decrease. Additionally, TEG systems have a high fuel consumption and the life span of a TEG system is typically less than 10 years.
A more recent development in providing reliable power to remote locations has been the introduction of fuel cells. A fuel cell is an energy conversion device that generates electricity and heat by electrochemically combining a gaseous fuel and an oxidant gas via an ion conducting electrolyte. The main characteristic of a fuel cell is its ability to convert chemical energy directly into electrical energy without the need for heat conversion (i.e., converting heat to electric or mechanical power optimized in accordance with the Second Law of Thermodynamics), giving much higher conversion efficiencies than heat engines (e.g., engine generators, CCVTs or TEGs). A system having such fuel cells and a gas turbine for achieving high efficiencies has been proposed by Siemens Westinghouse, as indicated in their website. However, the fuel cell technology is not mature and the life and reliability of the fuel cells are not sufficient to maintain reliable remote power without a proven backup for when the fuel cell fails.
Finally, the closed cycle vapor turbogenerator (CCVT) systems have a fuel consumption which, although lower than the TEG system, is much higher than that of a diesel generator. Redundancy for these systems is usually achieved through the use of one or two operating CCVTs; with one CCVT on warm standby. Fuel consumption varies in accordance with the load but the use of two CCVT each operating at half load consumes 20% more fuel than one load at 100% load. Usually the level of power production in remote locations is between 1-10 kW.
U.S. Pat. No. 4,590,384, the disclosure of which is hereby incorporated by reference, discloses a peak shaving power plant for utilizing a source of low grade heat comprising a Rankine cycle turbine having an organic working fluid utilizing heat from a low grade heat source, a generator driven by the turbine and having a generating capacity in excess of the capacity of the turbine, and a fast starting prime mover, such as an internal combustion engine, having a capacity that is less than the generating capacity of the generator. A selectively operable coupling connects the output of the fast starting prime mover to the generator so that, on demand, the fast starting prime mover can drive the generator providing peak power shaving in the amount of the capacity of the prime mover.
In U.S. Pat. No. 4,982,569, the disclosure of which is hereby incorporated by reference, a hybrid power plant is disclosed and includes an intermittently operable non-fuel consuming power generator, such as a photovoltaic cell array, or a wind generator, connected through a control-circuit to a battery for charging the same during operation of the power generator, and for supplying current to a time-wise, substantially constant, electrical load. In addition, the hybrid power plant includes an electric generator connected to a standby operable prime mover, such as a Rankine cycle organic fluid turbogenerator, for charging the battery and supplying current to the electrical load when the intermittently operable non-fuel consuming power generator is not operating. In the case of a photovoltaic array, this situation occurs at night so that the prime mover of the hybrid power plant can be started as it becomes dark.
It is therefore an object of the present invention to provide a new and improved method of and apparatus for providing ultra-reliable power wherein the disadvantages of high fuel consumption, unreliability, maintenance, use of batteries and the associated climate control (which increases the power consumption and maintenance, thus reducing the reliability) as outlined above are reduced or substantially overcome.